feat(usage): rename metrics → tokens_embedded/pages_embedded + export token cost to Prometheus

Billing product model finalized (Deck #281): bill pages externally, record
tokens internally. Rename the data-plane metric literals to match the now-
canonical contract (Deck #284) — the control plane's METRIC_EVENT_NAMES is
already renamed, so the old names would be unmapped and never sync to Stripe.

Rename (values unchanged):
- embeddings_queries → tokens_embedded (value = real token count, already
  emitted by this PR; the unit upstream providers bill on).
- pages_chunks → pages_embedded (value kept as len(chunk_texts) interim;
  TODO(#282): real normalized "pages indexed" count — real pages for paginated
  types, chars/tokens-per-page constant otherwise — is deferred to the
  instrumentation card, this only lands the name/contract).
- All literals, log strings, docstrings, comments, the migration comment, and
  tests renamed; grep confirms zero old strings remain.

Observability (new): export embedding token cost to Prometheus as
astrolabe_embedding_tokens_total{provider,operation} (operation = index|query)
so the billed cost unit is visible in Grafana, not just the per-tenant billing
DB. Dedicated counter (doesn't inflate the existing chunk/request metrics) and
always-on (independent of USAGE_METERING_ENABLED, so OSS/self-host gets it).
Wired on both the indexing batch embed and the search query embed (query inside
the per-request cache-miss branch, so reused embeddings aren't double-counted).

Note: the rename orphans any pre-existing embeddings_queries/pages_chunks rows
in tenant app DBs (CP no longer maps them) — acceptable; pipeline is inert with
throwaway dev/sandbox data.

Deck #284 (folded into PR #875).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/test_processor_metering.py

@@ -1,7 +1,7 @@
 """Unit tests for the indexing-path usage-metering helper (Deck #67).
 
-``record_indexing_usage`` records the two billable events (``pages_chunks`` +
-``embeddings_queries``) after a document's chunks are embedded. These cover the
+``record_indexing_usage`` records the two billable events (``pages_embedded`` +
+``tokens_embedded``) after a document's chunks are embedded. These cover the
 value mapping, the flag/zero-chunk no-ops, and the best-effort failure path
 without standing up the full document pipeline.
 """
@@ -25,8 +25,8 @@ def store_spy(monkeypatch):
 
 
 @pytest.mark.unit
-async def test_records_pages_chunks_and_token_count(store_spy):
-    """Both events fire: pages_chunks = chunk count, embeddings_queries = tokens."""
+async def test_records_pages_embedded_and_token_count(store_spy):
+    """Both events fire: pages_embedded = chunk count, tokens_embedded = tokens."""
     await processor.record_indexing_usage(
         enabled=True,
         provider="mistral",
@@ -40,7 +40,7 @@ async def test_records_pages_chunks_and_token_count(store_spy):
 
     calls = store_spy.record_usage_event.await_args_list
     by_metric = {c.kwargs["metric"]: c.kwargs["value"] for c in calls}
-    assert by_metric == {"pages_chunks": 110, "embeddings_queries": 4242}
+    assert by_metric == {"pages_embedded": 110, "tokens_embedded": 4242}
     for c in calls:
         # Hot-path fast-gate + tenant-local attribution metadata.
         assert c.kwargs["enabled"] is True